Monday, January 04, 2010

Burrr. Its a chilly, white winter here where I live and in want of some heat, I decided to bring some 'heat' into today's post! I'll talk a little about the cyclist's elements during hot weather cycling and will seek to understand what role certain elements of our kit, like helmets, have on cooling.

CYCLING IN HEAT

When a cyclist gets out for a ride during the pleasant summer months, the modes of heat transfer seen between them and the environment is of the following order. You might think it is easy to fathom, but it really isn't.

There's many packets of energy entering and exiting and bouncing off here and there, from the thermal radiation from the sun to the heat flux in the skin. Overall, what we see is a complex thermodynamic system in action that tries to approach homeostasis. Here's a diagram :

Some of these elements may be familiar to us. Taking a break after riding, you can feel the hot sun on your skin. Comfort and discomfort are associated with humidity. When the relative humidity and air temperature are both high, the rate of sweat evaporation happens more slowly and we sense a higher temperature than the actual temperature of the air. The temperature we ultimately feel is governed by the temperature-humidity index (THI), also called discomfort index.

Now there happens to be an easy formula (though there are several more complicated ones) you can use to calculate THI before your rides.

As the temperature gets higher, low %'s of humidity can result in an index that is much higher than the actual temperature. But the above formula does not take into account the reverse air cooling taking place when you ride your bike in a breeze.

Another aspect of the image above is the thermal radiation off the ground. Because of its dark color, roads happen to be good emitters and this infrared radiation varies proportionally as the fourth power of road surface temperature. You can get an idea of the energy emitted per unit surface area, or emissive power, by using the Stefan-Boltzmann law.

CLOTHING

With regard to the emission properties of colors and surfaces, you and I were taught from early years in school that light colors are always good in sunny weather for better reflection. Yet our sport is one of multitudes of color, dark color.

It always struck me as odd how we riders have managed to live with dark colors in hot weather. The saying has always been to choose fabrics with wicking properties. These days, companies like Campagnolo are trying to capitalize on the apparent discomfort of dark clothing by incorporating fibers with UV ray protection. It seems to me a majority of us don't really think about apparel's effect on heat transfer and prefer more to wear matching kits, including helmets. The predominant color for shorts among men and women is black, but that seems to be for obvious reasons.

HELMETS

Which brings me to the question of helmet color. Are light helmets better than others in terms of thermal behavior? And how many cooling vents are optimal? This must be an important question, after all the helmet sits on a cyclist's head and part of the body's internal temperature regulatory mechanism is contained inside the brain.

The question posed above cannot be easily answered. In 2006, a paper in the Journal of Sports Sciences from Bogerd et.al noted :

"Given the large variation in designs on the market, there is no widely adopted systematic approach to designing bicycle helmets for optimal ventilation. A quantitative survey of a large number of modern helmets is required to understand the role of "common sense" parameters, such as the number of holes in the helmet, since the helmet geometries are complex. In particular, the importance of forced, rather than natural, convection complicates a qualitative a priori analysis."

The paper referenced above isn't a bad read. It reports on the heat transfer, or forced convection cooling power, of an ensemble of 24 helmets. The study was done using an artificial headform. Apart from vent cross-sectional area and helmet channels, even the auxiliary elements of helmet design, such as the straps, and biological parameters like facial measurements and hair thickness have significant influence on ventilation properties, it seems. In the end, they concluded that "the wide variation in ventilation performance in the present ensemble serves to emphasize the lack of systematic understanding of the principles behind bicycle helmet ventilation."

As difficult the task above of studying helmet ventilation was, it didn't really seek to answer the question of helmet color and its effect on heat transfer.A simplistic thermal test between a black and a white helmet was done back in 2000 by Terry Morse of California. The test aimed to answer the question of whether or not a black helmet is hotter than a white one of same design when worn in direct sunlight, both while at rest and while moving.

Terry placed a temperature probe at the crown of a Styrofoam head form, and put the helmet on the head. He then hung a halogen lamp 5" above the helmet, turned a household fan on high speed (6.5 mph), & recorded the temperature every minute until it stopped changing. He then carried on the same test with the fan on low speed (5.0 mph), and finally with the fan off. He did these three tests thrice for a black and a white TREK "Vapor" helmet and a bare head situation.

What he found was interesting :

1. In the air cooled phase of the experiment, the bare head form showed the least temperature change (delta F) from ambient temperature while the head form with a black helmet showed the highest delta F. In fact as the airspeed was decreased, the black helmet's delta F kept increasing before it stopped showing any changes.

2. About 16 minutes after turning off the fan, the delta F in the base head form was fastest and highest. It was the most reluctant to stop gaining heat while the black helmet was the best emitter of heat, in other words, there was more cooling with a black helmet on. But Morse maintains that the heating rate was very close between both black and white helmets.

Graphs showing the temperature change (dF/dt) in high, low and zero fan airspeed. About 16 minutes after turning off the fan, dF/dt in the base head form was fastest and highest (green line). Click to zoom.

Studying the effects of helmet on head cooling is no easy matter. While the test above is simple in nature and a Styrofoam is certainly no human head, do you think it makes a reasonable conclusion that you're better off with something covering your head in conditions of little wind speed? What else would you consider when designing & selecting a helmet for cooling? Please feel welcome to discuss with comments.

13 comments:

"do you think it makes a reasonable conclusion that you're better off with something covering your head in conditions of little wind speed?"

I think it does. You go to parts in the middle east and the arabs who lead lives in desert environment know that they should be covering their head with a cloth. It seems to make sense. In a breeze, it's best to leave your head uncovered. When there's little cooling, it's better to leave the head covered. What's even better is pouring some water over your head, down your neck and most racers know this helps in cooling.

I am no engineer, but I live in the tropics and it is the hottest part of summer right now. I definitely think that I am cooler with my helmet on, I have a well ventilated Met helmet in a light colour, but the Styrofoam is so thick I don't think the outer skin colour matters. What does matter is the air flow over your skull. The helmet facilitates airflow while protecting your head from the direct rays of the sun.I am never too hot cycling, just so long as I don't stop moving!!

Everyone with any experience in the heat realizes dark-colored clothing is too hot. Wasn't everyone taught that in school?http://www.thisfabtrek.com/journey/africa/mauritania/20051225-nema/bedouin-femmes-child-4.jpg

Good input from you guys. This is what I thought too, that covering the head in heat and little airflow is best for cooling. Its interesting to see that before the day of the helmet, some racers in grand tours were climbing mountains without their helmet on (Hinault comes to mind). Did the conventional wisdom behind it say that's better for cooling?

I would also like to think wearing a helmet facilitates more cooling than just wearing a cycling cap.

DJConnel : The picture you attached seems to show two Arab (?) women in black dresses. I think heat transfer is not really the objective there asblack was used to cover their bodies and faces so that they wouldn't be exposed to the stares of men!! However, the men, if you'll notice mostly wear dazzling white robes. How convenient! They stay cooler and their female compatriots burn up in the heat! :)

[url=http://www.physicsforums.com/showthread.php?t=65287]The Bedouin example is an old one[/url].

The conclusion seems to be that if you're working hard, dissipating heat is important, and black is beneficial. But if you're sitting around, reflecting heat is more important, and white is better. I think male Bedouin's of higher status tend to wear white, while those doing the heavy lifting tend to wear black.

I was going some reading on the Arab traditional dress, the right name of which is "Thawb". I got this bit from a website on Arab clothing :

"Saudi men wear a full-length, loose garment with long sleeved called thawb. The normal color is white. It is made from cotton or a a polyester mix. However, this color is commonly changed into darker colors such as brown, navy blue, black during the winter. Winter's thawb are made from wool.The collar of the thawb comes in different styles. Some come in a rounded shape and others come in a triangular shape. Some of the thawb sleeves have a loose cuffs while others have tight ones. The tight cuffs are open and closed by placing a Kabak ( cuff buttons). Kabak comes in a variety of shapes and can be made of many different materials. Some are made by silver and decorated with diamonds or precious stones. Usually the style of the sleeve's cuff matches with the style of the collar."

So the colors for the Thawb depend on the weather. It may not have anything to do social status in society, although I have to admit a lot of those rich Sheikhs wear dazzling white and sometimes even cream colored Thawbs.

From a Physics point of view DJConnel is correct. Black bodies radiate and absorb heat best. So if your temperature is > ambient black radiates heat better. So you are generally around 100F so if is hotter than this black is better. At high altitude and dry air this is more important. Although in general much more heat is due to conduction than radiation.

Don't you think, most recreational cyclists do not exert enough energy to notice any difference, and might benefit from a helmet due to their inexperience in handling some unforeseen situations.

I cycle for fitness and train hard, my head without helmet suffers the heat less.

In your experiment you are ignoring the body uses sweat to cool down our body.

A helmet increases the relative humidity around the head, which according to the formula at the very beginning of your post, can increase perceived heat.

A hot head, even just the perception of heat, can be extremely dangerous, lowering responses and alertness.This increases a chance of accident rather than reducing it.

I know I am more alert at the end of a 100 mile ride without a helmet than with one.

On top of that, a helmeted head makes for harder work in windy situations, and the average helmet adds about 5% to the weight of a head, but around the outside. Which means it exerts a greater force on the neck muscles.

Watching the Tour and Giro, I also noticed that most helmets worn by pro ridersa) are the most expensive, lightest helmets you can find, which most people can't afford.b) are replaced as often as possible, something most people can't do, which means many riders are riding with helmets that would fail a test even though they can't see any external damagec) Most pro riders wear helmets extremely loose on their head to improve cooling, which makes the helmet much less effective in the event of a crash.

I wear a helmet when cycling on snow and ice, due to the chance of a slow speed unforeseen fall.

I also wear a helmet if cycling late at night due to the higher chance of falling due to unseen objects.

I also wear a helmet when cycling in very intense traffic.

But those account for maybe 25% of my total riding.

For the remainder 75%, such as cycling to the shops or on a training ride in the countryside, I feel safer without the helmet, and my alertness is higher throughout.

I have fallen many times, so I know there are chance of hitting your head, but those are smaller than the chances of being hit by a car while walking on the pavement (statistics gathered by the NHS).

Having said that, I am not recommending neither in favour or against wearing one.This is just my personal experience.

According to the above experiment it appears that if you were to ride your cycle with a ventilated helmet, your head would get slightly cooler the faster you go wearing a black helmet rather than a white helmet. However, when standing still the black helmet gets hotter to wear due to no ventilation and the heat from the dark collor radiates inward towrds the head.